1. Field of the Invention
The present invention relates to thermal printing, and more particularly, to a method and apparatus for coordinating transport rate and print rate of a print media within a thermal printer in order to compensate for top-of-form and image stretch errors.
2. Description of Related Art
In the field of bar code symbology, vertical bars of varying thicknesses and spacing are used to convey information, such as an identification of the object to which the bar code is affixed. The bar code symbols are typically printed onto labels having an adhesive backing layer that enables the labels to be affixed to objects to be identified. To read a bar code symbol, the bar and space elements are often scanned by a light source, such as a laser. Since the bar and space elements have differing light reflective characteristics, the information contained in the bar code can be read by interpreting the laser light that reflects from the bar code. Alternatively, a bar code symbol may be imaged by an electrically photosensitive element, such as a charge coupled device (CCD), and the individual bar and space elements identified from within a one or two-dimensional electronic image of the bar code symbol. In order to accurately read the bar code symbol, it is thus essential that the symbol be printed in a high quality manner, without any streaking, blurring or improper registration of the symbols to the labels. At the same time, it is essential that the adhesive backing layer of the labels not be damaged by heat generated during the printing process.
In view of these demanding printing requirements, bar code symbols are often printed using direct thermal or thermal transfer printing techniques. In direct thermal printing, the print media is impregnated with a thermally sensitive chemical that reacts upon exposure to heat. In thermal transfer printing, a thermally reactive ribbon is transported in parallel with the print media, and ink from the ribbon is transferred to the print media upon exposure to heat. Both of these printing techniques are referred to collectively herein as thermal printing.
Under either method of thermal printing, the print media is drawn between a platen and a thermal print head by a media transporting mechanism. The transporting mechanism may include stepper motors that transport the print media in small incremental steps of as little as five mils per step. The thermal print head has linearly disposed printing elements that extend across a width dimension of the print media. The printing elements are individually activated in accordance with instructions from a controller, which activates the thermally reactive chemical of the print media or ribbon at the location of the particular printing element. As the print media is drawn in a step-wise manner through the region defined between the platen and the thermal print head, the bar code symbols are printed onto the print media as it passes therethrough. Other images, such as text, graphics or symbols, can also be printed onto the print media in the same manner.
The print media may comprise a release liner onto which the successive labels are affixed. The release liner has a coating that permits the labels to be easily removed therefrom without adhering permanently, and which enables the labels to be effectively transported through the print region without sticking to various elements of the transporting mechanism. There is usually a small gap between adjacent ones of the labels which is used to separate the labels and to provide a guide for registration of the printed information to the labels, as further described below. Alternatively, the print media may comprise a non-adhesive card or tag stock having periodic indentations at opposite sides thereof which are joined by perforation lines. The indentations and perforation lines define gaps for registration of the printed information in a similar manner as the adhesive labels. Further, the adhesive or non-adhesive labels may also have pre-printed information, such as color graphics, that is intended to be aligned with the printed information applied by the thermal printer.
It is desirable within the art to maximize the amount of information printed on each label, and conversely, to reduce the amount of unused space on the labels. It is also desirable to avoid undesired misregistration of printed information to a respective label. To accomplish these goals, it is necessary to identify with a high degree of accuracy the leading edge of each label on the print media in order to synchronize the printing with the transport of the print media. Ideally, the gap between the adjacent labels is a constant width, however, in practice, there are inevitable variations in the gap width due to differences in print media production quality as well as stretching of the print media during its transport. For the printed information to begin as close as possible to the leading edge, it is necessary that the leading edge of the labels be identified within a single step size of the print media transporting mechanism. A discrepancy between the start of the printed information and the leading edge of the label is referred to herein as a top-of-form registration error.
As known in the art, gap sensor circuits are used to identify the gap between adjacent labels of the print media. A conventional gap sensor circuit comprises a photosensor having a light emitting element and a light receiving element. The photosensor is disposed relative to the print media with the light emitting and receiving elements positioned at opposite sides of the print media thereof so that light passes through the print media as it is transported. Since the non-gap regions of the print media will transmit less light than the gap regions, the gap can be detected by measuring the change in light transmissivity of the print media as it passes between the two photosensor elements. The conventional gap sensor circuits permit the thermal printer to limit top-of-form registration errors to a single step size of the print media. Even though the maximum top-of-form error is very small, however, it is still noticeable and is thus undesirable.
An additional problem relating to image registration is referred to herein as image stretch. As the size of the print media roll or thermal transfer ribbon changes from beginning to end, the amount of back pressure or drag applied to the media transporting mechanism changes in a generally corresponding manner. This variation in drag results in variation of the effective step size of the print media. Similarly, a step size variation may also be caused by changes in the mechanical systems of the printer, such as by replacement of the platen roller. Moreover, the slight differences in mechanical tolerances between otherwise identical printers will often result in non-uniformity of the step size. As the step size increases, the printed information may become elongated or distorted. These image stretch errors are not only aesthetically unsatisfactory, in some cases the image stretch may render the printed bar code symbols unreadable. Also, image stretch errors may further exacerbate top-of-form registration errors.
Thus, it would be desirable to provide a method and apparatus for closely coordinating the transport rate of a thermal print media with its print rate in order to precisely compensate for top-of-form and image stretch errors.